Coal, as mined, contains various forms of impurities including ash-forming minerals which form ash when combusted (hereinafter referred to as "ash-forming mineral matter") and inorganic (or pyritic) and organic sulfur. In order to make coal a more acceptable fuel, a significant proportion of the impurities must be removed. The conventional coal cleaning techniques, such as dense medium baths, cyclones, jigs and tables, can remove relatively coarse-grained impurities from coal without too much difficulty. However, the clean coal product from these techniques still contains a large amount of impurities typically in the 6-10% ash and 0.6-1.7% sulfur ranges. These impurities are finely disseminated in the coal matrix and, therefore require fine grinding before any separation technique can be applied to further remove them.
The objective of the grinding step is to liberate the mineral matter from the coal matrix. In some cases, the coal must be pulverized to micron sizes to achieve sufficient liberation. However, the micronizing is an energy-intensive process requiring sometimes 100 kwh or more of energy to pulverize a ton of coal. Furthermore, the size reduction frequently leads to a substantial fraction still containing "composite particles" made of both coal and mineral matter which are difficult to separate, resulting in a substantial loss of coal.
A micronized coal produced with such a large expenditure of energy is difficult to handle, and it is hard to clean it of its impurities. Many new fine coal cleaning processes have been suggested, including conventional froth flotation, microbubble flotation, selective agglomeration, selective flocculation, etc. Some of these techniques have been known to produce super-clean coal containing less than 1 or 2% ash and reduced sulfur. However, these processes suffer from relatively high consumption of reagents, difficulty in dewatering and generally low recovery, which are typical problems in processing fine particles. Chemical cleaning techniques oan produce super-clean coal from a relatively coarse coal, but it is intrinsically more expensive than the physical cleaning processes mentioned above.
The present invention suggests a new concept for cleaning coal of its mineral matter, including both the ash-forming minerals and pyritic sulfur. Meyers (U.S. Pat. No. 3,768,988) showed that pyritic sulfur can be removed substantially by treating the coal with ferric ions. In this process, the pyritic sulfur is oxidized at about 100.degree. C. to elemental sulfur and sulfate by the ferric ions, while the ferric ions are reduced to ferrous ions. In the Meyers process, the ferric ions are regenerated from the spent ferrous ions by blowing air or oxygen at a relatively high temperature. In a similar process, Lalvani et al. (1983) showed that ferric ions can be regenerated by an electrochemical method. Both Meyers and Lalvani showed that a significant amount of pyritic sulfur is removed from the coal, but neither of these processes showed any ash-forming mineral matter removal.